December 23, 2012

Day 23 - Down and Dirty Log File Filtering with Perl

This was written by Phil Hollenback (

Here We Go Again

Say you have a big long logfile on some server somewhere and a need to analyze that file. You want to throw away all the regular boring stuff in the logfile and just print lines that look suspicious. Further, if you find certain critical keywords, you want to flag those lines as being extra suspicious. It would also be nice if this was just one little self-contained script. How are you going to do that?

Sure, I know right now you are thinking "hey Phil just use a real tool like logstash, ok?" Unfortunately, I'm not very good at following directions, so I decided to implement this little project with my favorite tool: Perl. This post will shed some light on how I designed the script and how you could do something similar yourself.

Requirements and Tools

This whole thing was a real project I worked on in 2012. My team generated lots of ~10,000 line system install logs. We needed a way to quickly analyze those logs after each install.

This tool didn't need to be particularly fast, but it did need to be relatively self-contained. It was fine if analyzing a logfile took 30 seconds, since these logfiles were only being generated at the rate of one every few hours.

For development and deployment convenience, I decided to embed the configuration in the script. That way I didn't have to deal with updating both the script and the config files separately. I did decide however to try to do this in a modular way so I could later separate the logic and config if needed.

I had been previously playing with embedding data in my perl scripts through use of the DATA section so I wanted to use that approach for this script. However, that presented a problem: I knew I wanted three separate configuration sections (I will explain that later). This meant that I would have to do something to split the __DATA__ section of my file in to pieces with keywords or markers.

Naturally, I found a lazy way to do this: the Inline::Files perl module. This module gives you the ability to define multiple data sections in your script, and each section can be read as a regular file. Perfect for my needs.

Reading the Logfile

The first step in this script is to read in the logfile data. As I mentioned, it's around 10,000 lines at most, not a staggering amount. This is a small enough number that you don't need to worry about memory constraints - just read the whole file in to memory. With that, here's the beginning of the script:


use warnings;
use strict;
use Inline::Files;

my $DEBUG = 0;
my @LOGFILE = ();
my (@CriticalList, @WarnList);

  # skip comments
  next if /^#/;
  push(@LOGFILE, split("\n"));

What does that all do? Well of course the first few lines set up our initial environment and load modules. Then I define a LOGFILE variable. Finally the while loop uses the magic <> operator to take input lines from either STDIN or from any file specified on the command line.

Inside the while loop the combination of shift and push converts each line of input text into an array entry. At the end of this whole thing I've got an array @LOGFILE which contains the logfile data. As I mentioned, this works just fine for logfiles which aren't enormous.

Reading the Data

Next, I want to create arrays for three sorts of matches. First, I need a list of matches which we should warn about. Second, I need a list of matches that we ignore. The third list is a list of critical errors that I always want to flag no matter what.

The idea here is that the first list of matches is the broadest, and is intended to catch all possible anomalies. For example, the word 'error' is caught by the warn list.

The second list is more discriminatory, and acts to filter the first list. Any time a match is found on the warn list, it's checked against the ignore list and discarded if a match is found. Thus, the line

this is a spurious warning

is initially flagged by the warn list. However, the ignore list includes a match for 'spurious warning' so ultimately this line gets suppressed as we know it's spurious.

The final list short circuits the entire process - if any match is found on the critical list, no checking of the warn or ignore lists is done. This list is intended for only specific critical failures and nothing else. That way we can invoke a special code path for the worst problems and do things like exit with a non-zero value.

Remember that we are using Inline::Files so we can treat sections of the script as data files. Here's the end of the script that can be used to configure the run:

# nothing after this line is executed
# everything in this section will go in the critical array
.*File.* is already owned by active package
# everything in this section will go in the ignore array
warning: unable to chdir
set warn=on
# everything in this section will go in the warn array

We can now treat CRITICAL, WARN, and IGNORE as regular files and open them for reading like so:

open CRITICAL or die $!;
  next if /^#/; #ignore comments
  push @CRITICAL, $_;

Repeat for WARN and IGNORE. We now have three arrays of matches to evaluate against the logfile array.

Pruning the Logs

Now, we need to act on the log data. The simplest way to do this is with a bunch of for loops. This actually works just fine with 10,000 line logfiles and a few dozen matches. However, let's try to be slightly more clever and optimize (prematurely?). We can compile all the regexes so we don't have to evaluate them every time:

my @CompiledCritical = map { qr{$_} } @Critical;
my @CompiledWarn = map { qr{$_} } @Warn;
my @CompiledIgnore = map { qr{$_} } @Ignore;

then we loop through all the logfile output and apply the three matches to it. We use a label called OUTER to make it easy to jump out of the loop at any time and skip further processing.

OUTER: foreach my $LogLine (@LOGFILE)
  # See if we found any critical errors
  foreach my $CriticalLine (@CompiledCritical)
    if ($LogLine =~ /$CriticalLine/)
      push @CriticalList, $LogLine;
      next OUTER;
  # Any warning matches?
  foreach my $WarnLine (@CompiledWarn)
    if ($LogLine =~ /$WarnLine/i)
      # see if suppressed by an IGNORE line
      foreach my $IgnoreLine (@CompiledIgnore)
        if ($LogLine =~ /$IgnoreLine/)
          # IGNORE suppresses WARN
          next OUTER;
      # ok, the warning was not suppressed by IGNORE
      push @WarnList, $LogLine;
      next OUTER;

Output the Results

With that, the script is essentially complete. All that remains of course is outputting the results. The simplest way to do that is to loop through the CRITICAL and WARN arrays:

if (@CriticalList)
  print "Critical Errors Found!\n";

  while(my $line = shift @CriticalList)
    print $line . "\n";
  print "\n";

if (@WarnList)
  print "Suspicious Output Found!\n";

  while(my $line = shift @WarnList)
    print $line . "\n";
  print "\n";

Assuming a logfile like this:

1 this is a warning: unable to chdir which will be suppressed
2 this is an error which will be flagged
3 set warn=on
4 this is superfluous
5 set warn=off
6 looks like File foobar is already owned by active package baz

the script outputs the following:

$ log.txt
Critical Errors Found!
6 looks like File foobar is already owned by active package baz

Suspicious Output Found!
2 this is an error which will be flagged
5 set warn=off

The end result is exactly what we want - a concise list of problematic log lines.


This is a pretty simple example, but hopefully, it gives you some ideas to play with. As I said in the beginning, I realize that there are lots and lots of other more clever ways to go about this sort of log analysis. I won't claim that this is even a particularly smart way to go about things. What I can tell you is that a variation on this script solved a particular problem for me, and it solved the problem very well.

The thing I'm really trying to illustrate here is that scripting isn't that hard. If you are a sysadmin you absolutely must be comfortable with scripting. I prefer perl, but rumor has it, there are some other scripting languages out there. If you have the ability to throw together a script you can quickly and easily automate most of your daily tasks. This is not just theory. I used to review multiple 10,000 line install logfiles by hand. Now I don't have to do anything but look at the occasional line that the script flags. I freed up a couple hours a week with this approach and I encourage all of you to take the same approach if you aren't already.

Further Reading


Anonymous said...

I'd suggest take a look on another log analysis tool which rich functionality may cover all your needs - SEC(simple event correlator)
Also it's a good example of well written perl code.

Unknown said...

You might want to take a look at logstash and elastic search - pretty awesome handlers for doing parsing of logs and centralizing log stores. They've got a grok filter that works great :)